templates: document how to turn on the new node styles

Documents jj-vcs#3501, though
only in `templates.toml` for now (since this is mostly for
jj developers to experiment with for now).

Let me know if you prefer that I document this in config.md.

cli/src/config/templates.toml

@@ -166,6 +166,12 @@ if(hidden,
 )
 '''
 
+# Uncomment the following or put it in your config to enable facier graph node
+# styling.
+#
+# log_node = 'builtin_log_node'
+# op_log_node = 'builtin_op_log_node'
+
 builtin_log_node = '''
 label("node", 
   coalesce(